This invention relates to a power source for supplying a DC voltage to a load by converting an AC voltage from an AC line having low harmonic components of an AC current so that energy loss of the power source is very low.
An AC/DC converter which directly rectifies AC 100 [V] supplied from an AC line so as to provide a DC voltage of about 130 [V], so called a line operation type converter, is widely used as a compact DC power source. Japanese Patent Laid-open No. 1-252174 may be cited as an example of such apparatus.
In the converter as stated above, output DC voltage is controlled by changing a duty ratio of an ON/OFF time duration of switching elements Q1, Q2 which are connected to an output transformer so as to keep the output DC voltage constant. FIG. 6 shows wave forms in a rectifier for rectifying an input line AC Voltage (a). An input AC line current flows as shown in (b) of the FIG. 6 and peak value (b) of the current generated by switching the elements Q1, Q2 becomes three to five times the peak value of the effective current of the input AC current.
Such current contains many harmonic components such that there arises serious bad influences on the AC line or periphery instruments. In order to improve such problem, an apparatus as shown in Japanese Laid-open 1-152960 is proposed.
That is, switching element 14 for chopping the input DC voltage is operated with a high frequency more than several ten KHZ and a choke coil 13 is provided between the rectifier and the switching elements 4, 5, and the harmonics can be removed from the current as shown in FIG. 7. Therefore, the rectified DC voltage is chopped as shown in (b) and the current mixed with no harmonics flows as shown in (c).
But, in the improved apparatus, as the switching element and flywheel diode operated with the high frequency are needed, the efficiency of the apparatus becomes very low.